1. Field of the Invention
This invention relates to an improved method of and apparatus for casting solid propellant rocket motors.
2. Description of the Prior Art
High performance, solid propellant rocket motors require void and defect free grains in order to maximize total impulse and obtain stable performance. Entrapped air and volatiles must be removed from the propellant when cast. Conventional means of propellant deaeration include extrusion of the uncured propellant through a slit plate into an evacuated chamber or motor case. Entrapped air and volatiles are flashed from the ribbons of propellant exiting the slit plate.
A conventional slit plate, as illustrated in FIG. 1 of the drawings, contains parallel rows of narrow slits each of which normally has a width of one-sixteenth to one-quarter inch (1/16"-1/4"). This geometry provides a maximum cross sectional flow area. Laboratory tests conducted with such a parallel slit plate indicates that this geometry produces a non uniform propellant flow distribution. A typical flow pattern obtained with this geometry is illustrated in FIGS. 2A and 2B. Propellant ribbons leaving the slit plate stretch and elongate from their own weight. This results in narrowing or necking down of the ribbon thickness and width and the non uniform flow pattern illustrated in FIG. 2A. A non uniform flow pattern results in unequal filling of a rocket motor case and the formation of radially oriented interfaces and knit lines in the propellant grain. Such interfaces and knit lines are potential sources of grain failure.
Accordingly, there has existed a need and a demand for improvement in the art of casting solid propellant rocket motors to the end of avoiding and/or overcoming the aforementioned difficulties and problems.